Metal masks are generally used for a vacuum deposition process while manufacturing an organic electroluminescence (EL) device, an organic semiconductor element, etc.
Such metal masks have a three dimensional (3D) structure that has a plurality of circular holes or tapered structures. Semiconductor elements such as organic EL devices are manufactured by arranging the metal mask on a substrate and depositing a luminescence layer of a desired pattern to a specific area of the substrate.
U.S. Pat. Nos. 5,348,825 and 5,552,662 disclose conventional wet etching methods used to manufacture a metal mask, the methods include a chemical wet etching method of manufacturing a shadow mask. The shadow mask that is applied in present industrial sites is the chemical wet etching type.
A brief explanation of conventional wet etching is described below with reference to FIG. 1.
1. Resist coating: coating a photoresist 2 on both sides of a metal film 1.
2. Pattern coating: performing an exposure process on the photoresist 2 by using a glass mask pattern 3 (or quartz mask).
3. Developing: after the glass mask pattern 3 (or quartz mask) transcription on an upper surface of the photoresist 2, removing the glass mask 3 used for forming the pattern and selectively removing the photoresist 2 by performing a development process.
4. First etching: performing wet etching on the upper surface of the photoresist 2, in which the pattern is formed, to remove a part of a metal film 1 whereby the photoresist 2 is removed (aperture of the photoresist 2) by using an etching solution.
5. Filling: filling an anti-etching packing material in the upper surface of the metal film 1 in which the part thereof is removed. The anti-etching packing material is filled to protect a shape of the upper surface of the metal mask formed by the first etching while performing etching to a lower surface of the metal film 1.
6. Second etching: etching the lower surface of the metal film 1.
7. Removing: removing the anti-etching packing material and the photoresist, and finally obtaining the metal shadow mask.
The above process lists a typical process of manufacturing the metal shadow mask by using wet etching, and various modifications are developed from the above process. For example, the step “5. Filling” may be skipped, or both sides of the metal film are etched at the same time. However, the metal mask is generally manufactured by using the technique of chemical wet etching described in FIG. 1.
Wet etching has an isotropic characteristic as shown in FIG. 2. In other words, the metal film is removed while the metal film is identically affected by the etching solution in all directions from the aperture of the photoresist. Thus, a cross sectional shape of the metal film is formed to have a semicircle shape, as shown in FIG. 3. Thus, the metal mask that is finally formed on the metal film includes an aperture in which peripheries of the aperture (see the encircled portion in the drawing) are very thin.
Thus, the thinness of peripheries of the aperture may have a bad effect on precisely and stably securing a size or shape of the aperture.
For this reason, wet etching of the metal mask is not generally performed on one side (upper surface or lower surface) of the metal film, but performed on both sides of the metal film as shown in FIG. 3. Wet etching is performed on both sides of the metal film by using various methods that are disclosed in U.S. Pat. Nos. 5,348,825, 5,552,662, etc.
A cross line (cross point in a cross sectional view), in which a mask formed on the upper surface and a mask formed on the lower surface intersect to each other, is formed by using conventional methods. In addition, the metal mask including a taper structure (32 in FIG. 3) with a small size may be implemented by performing wet etching with a weak strength on any one side of the metal mask. Size and shape of the aperture may be secured by using such a taper structure. For this reason, heights of undercut (t in FIG. 3) are claimed to be 30˜40% of the entire thickness T of the metal mask in the prior art.
However, such a taper structure is formed by the isotropic characteristic of wet etching, thus it may be formed to have the undercut shape.
The metal mask having such an undercut shape shows its limit when depositing an electroluminescence material to a substrate of display devices by using such a metal mask. When depositing the electroluminescence material through the aperture of the metal mask, the electroluminescence material is not uniformly deposited on the substrate due to the undercut shape.
In other words, the undercut shape causes a gradual deposition of the electroluminescence material on a position of the substrate corresponding to the undercut shape. As a result, a performance degradation of the display device occurs when manufacturing by using such a metal mask.
Meanwhile, it is known that at present wet etching may be applied up to 300 ppi (pixel per inch). However, it is difficult to use conventional wet etching methods to produce display devices having resolutions of QHD (approximately 500 ppi) or UHD (approximately 800 ppi).
FIG. 4 is a view of explaining an isotropic shape of conventional wet etching (formulas of (1), (2), and (3) show correlations between factors of the shape (A, B, D, E, T, pitch, and Etch factor)), and through interaction formulas between factors of the shape.
The limit of implementing high resolutions of display devices when using wet etching can be explained. Thus, the figure does not show wet etching that is performed on both sides of the base.
Generally, the higher the resolution that is required, the smaller the value of pitch in FIG. 4 that is required, and thus a value of width B should also be smaller. According to a formula (3), in order to get a smaller value of width B, a smaller value of PR width A or depth D is required.
However, the value of PR width A cannot become an infinitely small value because it is difficult to obtain a very small value of PR width A due to the characteristic of an exposure process. Although an infinitely small value is obtained, it may cause degradation of an etching factor.
In addition, there is also a limit to set the depth value D to a small value. This is because, although the method of etching both sides of the metal mask is used, referring to FIG. 3, the size of the undercut becomes larger when the depth D value becomes smaller, thus the electroluminescence material is not uniformly deposited on the substrate. Further, a thickness T of the metal mask cannot be decreased since there is a minimal thickness required to handle the metal sheet during wet etching.
In addition, it is also difficult to implement display devices with high resolutions by only performing wet etching. The reason may be found on fine structures shown in a top plan view.
The isotropic characteristic of wet etching is shown not only in the cross sectional shape of the mask but also in the top planar shape of the mask. As shown in FIG. 5, an actual processed 3D shape of the mask has a bowl shape, thus four edges of the mask are rounded and not sharp. Such characteristics are characteristics that are difficult to be applied in display devices that require sharp quadrangular or polygonal deposition areas. In particular, it is difficult for such characteristics to be applied in display devices with high resolutions such as QHD or UHD.
Therefore, it is difficult to implement display devices having resolutions of QHD (approximately 500 ppi) or UHD (approximately 800 ppi) by using conventional wet etching due to limits and correlations between factors of the shape described above.
Meanwhile, recently, a metal shadow mask is manufactured by using an ultrashort pulse laser. Korean Patent Application Publication Nos. 10-2013-0037482 and 10-2015-0029414 are typical techniques, and the applicant of the present invention has also filed applications for the related inventions (Korean Patent Application Nos. 10-2014-0182140 and 10-2015-0036810).
FIG. 6 is a view of showing a basic process of manufacturing a metal shadow mask by using a laser.
A method of manufacturing a metal shadow mask by using laser includes:
1. a first irradiating step of irradiating a laser beam onto a substrate while moving the laser beam along a first looped curve that corresponds to a shape of mask hole; and
2. a second irradiating step of irradiating the laser beam onto the substrate while moving the laser beam along a second looped curve that is provide inside the first looped curve and has a smaller internal area than that the first looped curve.
3. In addition, another method of manufacturing a metal shadow mask by using laser includes: a first irradiating step of irradiating laser beam having a first energy onto a position in which a mask hole is formed on a substrate; and a second irradiating step of irradiating the laser beam having a second energy that is lower than the first energy onto the same position onto which the laser beam of the first irradiating step is irradiated.
The method of manufacturing a metal shadow mask by using such a laser, in order to improve an accuracy of the processed metal mask, generally uses an ultrashort pulse laser. The metal base is gradually removed or processed by the accumulation of various low-intensity pulses by using the ultrashort pulse laser.
Such method using the laser has an effect of specifying an energy distribution or intensity of the laser beam irradiated onto the metal base by configuring a specific optical system or changing the intensity change of the laser or pulse modulation.
For example, it may be possible to manufacture a metal mask having a proper taper structure without including undercut by configuring an optical system having specific energy distribution and controlling the laser and a relative motion of the substrate (Refer to FIG. 7).
However, the biggest limitation of the above method is that it is difficult to ensure productivity capable of being used in industrial sites.
In other words, the metal processing method using the laser continuously applies energy to the metal base in pulse train of the laser and induces a removal of the metal material of the base that is gradually removed from the surface of the metal base. Herein, processing speed (amount of removed material) may be increased by applying increasing the intensity of the laser irradiated to the metal base. However, heat due to the high energy applied to the metal base cannot be sufficiently dissipated and is accumulated on the metal base, thus the accumulated heat causes degradation of processing quality.
In addition, the high energy pulse of the laser applied to a surface of the metal base causes burrs on the other surface of the metal base. The energy pulse of the laser is applied to the metal base, gradually processes the mask and induces a shape that passes through the metal base. However, when the metal base is mostly removed and the metal base has very thin thickness, just before the mask passes through the metal base, then an impact of the high energy pulse works as a force to protrude through the metal base to the opposite surface. In case of invar material, heights of burrs may be from several microns to several tens of microns from the back surface of the mask.
When the organic luminescent material is deposited by using the shadow mask including such burrs formed on the back surface of the mask, glass damage may occur since the shadow mask is not completely stuck to the glass and thus the shadow mask sticks out of the glass. As a result, deposition performance is degraded by the shadow effect.
In conclusion, in order to ensure a high quality shadow mask, the metal material should be gradually processed by applying multiple laser pulses having a minimum energy required for the processing. However, it is difficult to ensure sufficient productivity by using such method.